Really ? Couldn't it be simply a regular Italian plural,
where final «-o» becomes «-i» ?
Thus scenario, using this standard Italian plural rule,
becomes scenarii.
My Italian is a bit rusty but I believe there are quite a
few of these plurals for word ending in -io :
ronzio/ronzģi, stantio/stantģi (stressed i) or the optional
(for unstressed i) desiderio/desiderii/desiderī/desidčri, it
must be said, if I recall properly, that for unstressed i
the plural is usually simply i (studio/studi).

I see no trace of latin in this.

> similar to the use of "viri" or (worse) "virii" as plural
of "virus".
> In Latin, "virus" (meaning "poison") was a mass-noun with
no usual
> plural form;

Yes, I believe this is true. Vīrī would be the expected
nominative plural but Perseus does not seem to list any.
Just found this in Allen & Greenough Latin Grammar :
«
The following in -us are Neuter; their accusative (as with
all neuters) is the same as the nominative: pelagus, sea;
virus, poison; vulgus (rarely M.), the crowd. They are not
found in the plural, except pelagus, which has a rare
nominative and accusative plural pelage.
»